1. Field of the Invention
The present invention relates to stepping motors, and more particularly to structures of stepping motors and synchronous motors that may be mounted in closed narrow space in various office automation (OA) equipment, audio visual (AV) equipment, robots and cameras, and applied to automatic control systems for positioning, focusing and other operations
2. Description of Related Art
A stepping motor is generally composed of a rotor section, and a stator section including a case, coils, a bearing section and a terminal section. A body diameter of the stepping motor (hereafter referred to as a xe2x80x9cmotor widthxe2x80x9d) perpendicular to a rotor shaft is determined by the outer diameter of the case. However, since the motor width is determined by the outer diameter of the case, an available maximum outer diameter of the coils is given by subtracting a plate thickness of the case and an insulation gap between the coils and the case from the outer diameter of the case. Accordingly, when environments where the motor is disposed have certain restrictions on the motor width, the coils cannot be wound in a number that effectively satisfies the motor rating. Furthermore, when the length of the motor in the axial direction is restricted, the coil volume is affected, and the torque characteristic is lowered.
To address the problem described above, a stepping motor in which magnetic pole pieces are not formed on yoke plates on both sides thereof in a specified diameteral direction is known. Notches are cut in the yoke cases along two parallel lines on both sides of the rotor, to thereby shorten the size in a specified direction. In this case, the reduction of the motor in size is attempted on condition that the diameter of the rotor and the number of coil windings should not be reduced. Therefore, if plane sections, which may correspond to chords of arcs, are cut to form the notches in the yoke cases while maintaining the air gaps without reducing the thickness of the coils, a plane of the rotor intersects the notches. Therefore, in order to reduce the size of the motor, the magnetic pole pieces inside the coils corresponding to the notches in the yoke cases need to be removed. In other words, in accordance with the motor described above, the coils are modified on the inside to thereby attempt to reduce the size on the outside.
However, because the magnetic pole pieces are not formed in the direction in which the sized is reduced, the magnetic field around the entire periphery of the rotor becomes irregular, which affects the motor characteristics including, for example, the frequency characteristic and torque characteristic.
It is an object of the present invention to provide motors including a stepping motor and a synchronous motor, in which the motor width in a specified direction is shortened, a uniform magnetic field is achieved around the entire periphery of the rotor, an effective number of coil windings can be secured, and a large torque is obtained with small current.
A motor in accordance with the present invention is equipped with a rotor equipped with a magnet, a stator yoke having claw poles with plural comb-teeth like pole teeth disposed opposing to one another and arranged in a cylindrical shape extending in an axial direction and around an outer periphery of the rotor in a circumferential direction, a bobbin formed from a coil wound around the stator yoke, and a cylindrical motor case that encircles the rotor, the stator yoke and the bobbin and formed from conductive material, wherein the motor case has side wall sections defining plane opening sections in areas opposing to portions of the bobbin, and an outer peripheral surface of the coil of the bobbin is protruded from the opening sections. As a result, the amount of coil windings is increased.
Also, an outer peripheral surface of the coil of the bobbin may be located in close proximity to the plane opening sections on the side walls of the motor case. Normally, the outer peripheral surface of the coil of the stator bobbin needs to be spaced a specified distance from an internal wall of the conductive motor case that defines the outer shape of the motor to secure an insulation gap, and therefore cannot be placed closer than the specified distance to the inner wall of the motor case. However, since portions of the side walls of the motor case are cut out, the motor becomes free of the problems relating to the insulation gap, and the winding section of the stator bobbin can be increased to an area in close proximity to the opening sections, where the side walls of the motor case occupied in the conventional motor structure.
Furthermore, the side wall sections of the motor case are provided with bent sections that are discontinuous in the circumferential direction and formed adjacent to edge sections of the plane opening sections. The bent structure in the side wall sections improves the rigidity in areas near the edge sections of the opening sections, whereby the precision of the motor structure can be readily maintained. As a result, the coil winding section can be placed closer to the cut sections, and may be protruded out of the motor case through the cut sections, such that the amount of coil winding can be increased. Furthermore, the motor case may be formed from magnetic material and also made to function as the stator yoke. As a result, common parts can be used for the motor case and the stator yoke, and therefore the number of parts can be reduced.
Other features and advantages of the invention will be apparent from the following detailed description, taken in conjunction with the accompanying drawings that illustrate, by way of example, various features of embodiments of the invention.